Lights aren't so bright

Confusing symptoms and misleading markings lead to a lengthy fault finding process for what turned out to be a simple problem.

The problem was simple: One headlight in the car was brighter than the other. My father, who is a mechanic, immediately diagnosed the problem as a poor ground. He connected a new wire from the terminal marked ground on the light, but this only made a marginal improvement. My brother then knew that it must be a faulty wire and ran a second wire from the switch to the high beam positive. However, this had no improvement. Connecting it to the low beam didn't result in any improvement. The light itself operated fine when a positive wire was run directly from the battery, eliminating it from the problem.

It was then my turn to have a go at it. I decided to measure the voltages of the good light, and compare them to that of the bad light. However, when I disconnected the good light, the bad light went out. When I disconnected the bad light, it had no influence on the brightness of the good light. I knew that the lights were wire up in parallel so the good light should have no influence on the bad light. It was as if the good light was behaving as a load in parallel, but the bad light was acting like a load in series.

I then decided to ground the ground terminal on the good light to make sure it was operating normally. However, when I did this, I blew the fuse. This should have been enough for me to work out what the fault was but, at the time, I was still at a loss.

I then decided to check the operation of the switch. I got my father to disassemble the steering wheel cover so we could access the switch. Upon inspection, it appeared that the switch was switching the negatives to the ground, rather than the positives to the light.

I now had the feeling I knew what the problem was. I spent a quarter of an hour drawing up the circuit diagram of the lights. The only way I could make it work, and replicate the fault behavior, was if the ground was actually +12V and the high and low beams were both ground.

This explained why the lights were behaving like they were. With the positive terminal (marked ground) grounded on the bad light, a small amount of electricity was flowing from the good light to the bad. This was because the path to ground was of lower resistance through the filament than through the switch. Of course, as the light brightened, the resistance would increase, sending more through the switch. This is why the lights were also dim.

The solution was simple, I just ran another wire off the +12V from the head light circuit to the “ground” terminal of the bad light.

I had mixed feelings about finding the solution to this problem. On the one hand, I did solve it, on the other, I could have solved it a lot quicker, had I realized what the symptoms meant.

The car was a Daihatsu Charade, a 1990 model. The lights did work by switching the ground connection, but I guess that the lights were built for positive supply switching since they were fed with +12 V to the ground connection and the high and low beams went to the ground.

Talking of strange car wiring faults and quality control, on one occasion the fault showed itself as a dimming of the clock display when the brake was applied. After much head scratching and work I finally found that one of the double filament rear lights had a short between the filaments inside the bulb. I never quite worked out why the clock display was affected, but replacing the bulb fixed it.

I had a case a while back where both low beams were intermittent. The high beams always worked but sometimes the low-beams didn't. It was all the more difficult to troubleshoot because sometimes just turning the car off was enough to cause the lights to start working again. Eventually, I was able to verify that the switch was working, even when the lights were not.
What really confused me was when I both verified that the terminals at the lights had a good 12V and both grounds seemed to be pretty darn close to zero ohms.
Eventually, just for kicks, I replaced one of the headlights. Suddenly, only one headlight was intermittent. I replaced the second one and the problem went away completely.
Looking closely at the bulbs, I could see that both had a broken filament that would lay down and make physical contact but could easily be jostled loose. Somehow, I had broken both filaments at the same time in the same manner. I still find that odd and that both went off and on at the same times.

ON the car with the headlights problem, I wonder if the headlight switch was originally intended to be switching the ground return for the lights. I can see that it would take less wire to control them that way, and it could avoid the problem of rusty ground connections at the lights. It would be interesting to know what make of car it was.

Liketobike is certainly correct about quality. Each generation of product is less robust and dependable, with new features that are useless and performing the intended task less adequately than the previous generation. So I repair the old one because I don't want the new one because it does not provide the function that I am looking for. Progress is seldom improvement in this era.

Poor grounds have been wreaking havoc in cars forever. In my first car, a 1970 model, all the lights would occasionally dim when I hit a bump or moved the auto trans shift lever. I searched all over for the culprit, using the vehicle schematic (all one page of it) for help. I finally found the problem working under the hood at night when I saw sparks jump across the well-worn transmission shift linkage! Since the engine, trans, rear suspension, and exhaust all mount with rubber bushings, a failed ground strap from the motor to the body caused the entire drivetrain to be isolated from the chassis. A simple fix after weeks of fruitless troubleshooting.

I must be entering old-fart-hood :-) Half the time at least, I want the old but still-working gizmo because I am more familiar with it, or it works better, or the new one omits some feature I want, or maybe I just have a nostalgic attachment to it (e.g., it belonged to a family member).
Maybe this ties into the quality thread :-)